(A) Field of the Invention p The present invention relates to a wafer lithographic shielding mask and a wafer lithography method using the same, and more particularly to a lithographic shielding mask for multi-project wafers (MPW) and a wafer lithography method using the same.
(B) Description of the Related Art
According to current techniques for fabricating MPW, layout patterns of several integrated circuits or chips from different projects are disposed in one reticle, so that the tooling cost of the reticle is shared by individual projects. The layout patterns for each chip refer to those required to be formed on the reticle for producing a working chip. However, as chips of various lengths or widths are placed in one reticle, in order to reduce the tooling cost of the reticle and the production cost of wafers, the problems of floorplanning and dicing of MPW arise. As to the floorplanning, the positions of the chips on the reticle must be determined. As to the dicing, the dicing plan which refers to a set of sawing lines being used for obtaining bare dice from a wafer is required before fabricating wafers, so as to determine the required number of wafers and facilitate wafer sawing after fabricating the wafers.
FIG. 1 shows a conventional device for a lithographic process; an MPW reticle 101 is disposed between a lithographic light source 102 and a projection lens 103. The light beam emitted from the lithographic light source 102 passes through the MPW reticle 101 and becomes a patterned light beam, and then is demagnified by the projection lens 103 and irradiated onto the photoresist surface of a wafer 104. The arrow on the surface of the wafer 104 indicates the stepping direction of exposure. Though the tooling cost of the reticle 101 of the MPW can be shared by individual projects, comparing the MPW reticle with the conventional reticle which consists of one or several copies of the layout patterns of a single chip, the cost of the lithographic process for the MPW and the cost of fabricating the required wafers are greatly increased. The main reason is that the diamond blade widely used in wafer sawing can only cut a wafer in a straight line from one side of the wafer to the other side of the wafer. Due to this limitation, sawing a certain die out of a wafer may frequently destroy many other dice in the same wafer. Conventionally, the MPW reticle is employed to fabricate chips for special projects with low-volume production requirements. As the tooling cost of reticle increases dramatically with advanced semiconductor process technology, the potentiality of using MPW reticle for larger volume production also increases considerably. However, because different production volume requirements exist among the projects participating in an MPW fabrication run, in order to produce enough bare dice for the projects with considerably large production volumes, excessive number of wafers will be fabricated. These wafers produce unwanted dice for the projects with smaller production volumes. In view of the above, the tremendous increase in the fabricating cost of the MPW is caused by the fabrication of some unwanted dice or dice destroyed by wafer sawing.
Once the dice for a certain project participating in an MPW run pass manufacturing tests, the project may proceed to a volume production. There exist two approaches to making a volume production for the project. The first approach is to reuse the MPW reticle for wafer fabrication. This is expensive and inefficient because this approach will also fabricate the dice not belonging to the underlying project. The second approach is to make an exclusive reticle for the project itself, instead of using the MPW reticle. This is also very expensive if the production volume is not large enough to justify such an investment. Therefore, a cost effective method is yet to be introduced.